There are numerous surgical situations in which bone grafts are used as part of the surgical procedure. For example, bone grafts are used in facial reconstruction, in repairing long bone defects, and in spinal surgery such as intervertebral discectomy and fusion in which a bone graft implant replaces a spinal disc in the intervertebral space.
Bone used for bone graft implants is often removed from another portion of a patient's body, which is called an autograft. A significant advantage of using a patients own bone is the avoidance of tissue rejection, but harvesting bone also has its shortcomings. There is a risk to the patient in having a second surgical procedure (bone harvesting) performed at a secondary site which can lead to infection or additional pain to the patient. Further, the bone harvesting site is weakened by the removal of the bone. Also, the bone graft implant may not be perfectly shaped which can cause misplacement of the implant. This can lead to slippage or absorption of the implant, or failure of the implant to fuse with the bone it is in contact with.
Other options for a bone graft source is bone removed from cadavers, called allograft, or from an animal, called xenograft. While these kinds of bone grafts relieve the patient of having a secondary surgical site as a possible source of infection or pain, this option carries a high incidence of graft rejection and an increased risk of the transmission of communicable diseases.
An alternative to using living bone graft implants is the use of a manufactured implant made of a synthetic material that is biologically compatible with the body. With varying success, several synthetic compositions and various geometries of such implants have been utilized. In some instances, the implanting surgery of such implants is accomplished without difficulty, but the results can be unsatisfactory because any minor dents or imperfections in the implant can cause poor bone-to-implant bonding or an implant having a very high porosity can collapse due to lack of mechanical strength. In other instances, the artificial implant requires a complex surgical procedure that is difficult to perform and may not lead to correction of the problem again, because of the above discussed side effects or dislocation of the artificial implant. Presently, no fully satisfactory artificial implant is known that can be implanted with a relatively straight-forward procedure.
Considerable study has been devoted to the development of materials that can be used for medical implants, including load-bearing implants, while allowing ingrowth of new bone tissue into the implant. To be suitable for this use, the material must meet several criteria, namely biocompatibility, porosity which allows tissue ingrowth and a mechanical strength suitable to bearing loads expected of natural bone without greatly exceed the natural bone's stiffness.
Several materials have been examined as potential implant materials including ceramics, such as hydroxylapatite, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2, hardened polymers and biocompatible metals. Hydroxylapatite (HAp) has been of particular interest because of its similarity to natural bone mineral, but it has only been used for low load bearing applications as pure porous HAp itself is relatively low in mechanical strength and may not serve as a good prosthetic material for high load bearing implants.
Studies have been directed at improving the mechanical strength properties of an HAp material in order to render it suitable as a high load bearing prosthetic material. European patent EP 0577342A1 to Bonfield et al. discloses a sintered composite of HAp and a biocompatible glass based on CaO and P.sub.2 O.sub.5 that may be used in dental and medical applications as a replacement for unmodified HAp. To date, improvements in the mechanical strength of HAp material has been achieved at the expense of its porosity. Upon densification necessary to achieve adequate load bearing strength, the HAp material has a porosity which is insufficient to provide the desired degree of bone ingrowth.
In a study entitled "Dense/porous Layered Apatite Ceramics Prepared by HIP Post Sintering," Materials Science, Vol. 8, No. 10, pp 1203 (October, 1989), by Ioku et al., the preparation of layers of dense HAp and porous HAp from two different types of HAp powder is discussed. This structure is prepared by first densifying specially produced fine crystals of HAp with a post-sintering process employing hot isostatic pressing (HIP). Then a commercial, coarse HAp powder is molded in layers with the densified HAp. Despite its being of academic interest, this type of HAp structure is not suitable for fabrication into load bearing bone prosthetic device configurations in which natural bone ingrowth may be achieved because of its lack of strength. However, Ioku suggests that the addition of zirconia whiskers into the dense HAp layer might provide some of the toughness necessary for hard-tissue prosthetic applications.
Still desired in the art is an artificial bone graft implant that is formed of a biocompatible mineral material similar to bone which possesses compressive strength close to that of natural bone while providing for ingrowth of bone tissue for permanent fixation.